This invention is concerned with variable displacement pumps which are used to power and control hydraulic systems.
In a simple hydraulic system, a pump draws oil from a low-pressure reservoir and supplies it at high pressure to a consumer unit (s) such as a ram. The only losses in this system are due to leakage etc., in the pump and ram, and viscous loss in the pipes, but the ram speed is directly related to the pump speed.
As the fluid volumes demanded by the consumer unit(s) will usually be variable, a common way of controlling such a system is to use a controllable bypass, which returns a proportion of the pump output to the reservoir without going through the ram. The speed of the latter can clearly be varied from zero, with the bypass fully open, to the maximum speed, with the bypass completely closed. However, this is very wasteful of energy. In a second form of control, a series valve is located in the high pressure supply, but this is just as inefficient. The valve raises pump pressure above that actually required, thereby wasting energy. At higher pressures, leakages within the pump become more significant, so they act as a bypass, to control the speed.
While the speed of the simple system could be controlled by varying the speed of the pump drive, this is usually impractical, since the drive is either a constant speed electric motor or an engine with a limited speed range. Even if the speed could be varied, the control available could be very slow.
Conventionally, this problem is solved by the different forms of variable displacement pumps. Usually, these are piston pumps, in which the piston stroke is selectively variable by a swash-plate or eccentric, so that the amount of oil delivered per stroke is varied. The pump output can therefore vary independent of the speed of the prime mover. Unlike the systems previously referred to there are no losses caused by bypass or throttle valves.
Conventional variable displacement pumps are reliable and efficient. However, all of them need very high forces to move the swash plate or the eccentric, and an auxiliary power system, usually hydraulic, must be provided for this purpose. This increases the complexity and cost of the pump. Furthermore, because it is obviously undesirable to use a great deal of power to control the pump itself, the response is usually relatively slow. Control by electrical signals requires a further stage, such as electro-magnetic valves. These shortcomings have severely restricted the range of use of variable displacement pumps.